High revision rates have been widely reported for metal-on-metal hip resurfacing (MOM-HR), and subsequently the use of MOM has become a major public health concern subject to widespread coverage in both the scientific and lay press. High revision rates have been partly attributed to the release of metal debris from the bearing surfaces. High wear, occurring as a result of edge loading, has been associated with devastating inflammatory soft tissue reactions.[2-4] These patients have poorer outcomes after revision surgery.
Previous studies reported a significant association between cup inclination angle and both wear rates and blood metal ion levels.[6-8] However, the correlations with cup inclination are only weakly positive and therefore can not alone explain all the variation in wear rates and ion levels.[6, 8] Indeed, edge loading in MOM-HR is likely attributable not only to component position, but also component design[9, 10] and patient activity patterns.
The Articular Surface Replacement (ASR; DePuy Orthopaedics) is widely recognized as having the poorest clinical and wear performance of the contemporary MOM-HR designs.[1, 12, 13] Design features such as the reduced “arc of cover” (the angle subtended by the articular surface of the cup) and head-cup clearance are thought to increase the likelihood of edge loading and high wear.[12, 13] An overview of the design variations among manufacturers is shown in Table 1.
Table 1. Design Specifications of the MOM-HR Devices Included in this Study
|Prosthesis||Radial Clearance (μm)||Arc of Cover (°)||Manufacturing Method (as-Cast, or Wrought)|
|Adept resurfacing system (Finsbury Orthopaedics)||86.37||160||As cast|
|ASR hip replacement (DePuy Orthopaedics)||49.47||146–152||As cast (plus heat treatment)|
|Birmingham hip resurfacing (Smith and Nephew)||105.10||159–163||As cast|
|Durom™ hip resurfacing (Zimmer)||97.67||165||Wrought|
|Cormet hip resurfacing system (Corin)||68.23||159–165||As cast (plus heat treatment)|
Many of the variables that affect wear, both surgical and design, likely do so by moving the contact area between the femoral and acetabular bearing surfaces closer to the cup rim. This distance has been described in the literature , and in this study we have termed it the ‘contact patch to rim distance’ (CPRD). As shown by the two-dimensional schematic (Fig. 1), a reduction in this distance increase the likelihood of edge loading and high wear.
Figure 1. 2D schematic illustrating key surgical- and design- specific variables contributing to the contact patch to rim distance (CPRD).
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No previous studies have reported the relative contributions or effect sizes of the relevant surgical and design variables (including CPRD) on wear or ion levels. Calculating CPRD will likely reflect the complex 3D geometric interaction of multiple variables, and calculating this for individual patients may improve wear prediction, allowing clinicians to better stratify their patients for long-term follow-up.
There were two parts to our study. First, we developed a geometric model to calculate CPRD and determined the contribution of five surgical and design variables to this measure. Second, we calculated patient-specific CPRDs for 165 MOM-HR retrieval cases. We then determined the contribution and effect size of CPRD and other commonly reported clinical and design variables, on wear rates and ion levels. Finally, we determined the predictive value of a low CPRD for raised ion levels and high wear rates.
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- Supporting Information
Our study contributes to understanding the wear performance of MOM-HR. To our knowledge, this is the first to quantify the contribution and effect size of specific surgical and design variables on bearing surface wear rates and blood metal ion levels from retrievals, and also the first to report the associations with CPRD as calculated from individual patient and implant data.
A previous multivariate analysis of MOM hip retrievals identified the presence of edge loading as the main predictor of bearing surface wear rates. Other studies identified the angles of cup inclination and version as significant predictors of wear, and speculated that design variables, such as arc of cover[9, 10] and clearance, may explain the variation in wear performance among MOM-HR designs. These variables are all thought to influence wear performance in the same way, by affecting the susceptibility of the hip to edge loading. Edge loading occurs when the contact area between the head and cup intersects the rim of the cup. Therefore, it is likely that CPRD can determine the susceptibility of a MOM-HR to edge loading and is likely to correlate well with component wear and blood metal ion levels.
Using our 3D model to calculate CPRD, we evaluated the relative contributions of five variables on the risk of edge loading. Surgical position of the cup, a combination of inclination and version angles, was the most important determinant of CPRD. The head diameter contributed significantly to CPRD, and this may help explain why small diameter MOM hips (i.e., those used in females) have higher revision rates. Of the component design variables, “arc of cover” contributed more to CPRD than clearance.
We then showed that surgical positioning of the cup also contributed more to high wear rates and metal ion levels than differences in component design. Combined, cup inclination and version explained an average of 21% of the variability in wear rates and ion levels. In comparison, the design variables tested (head diameter, clearance, and arc of cover) explained on average 14%. Although this supports inclination as the most important variable associated with high wear, our results suggest that both suboptimal position and component design contribute significantly. It is clear that cup inclination alone is a poor predictor of bearing surface wear and blood metal ion levels.
Of the design features investigated, reduced arc of cover was most significantly associated with increased wear rates and higher ion levels, supporting evidence in the literature that the ASR hip resurfacing device, with a relatively shallow arc of cover, is significantly higher wearing than other designs.[12, 13] From our theoretical CPRD calculations for different MOM-HR designs, we showed how the ASR is more susceptible to suboptimal cup position. Even standing, a patient with a 50 mm diameter ASR hip resurfacing with a cup implanted within Lewinnek's “safe zone,” may be subject to edge loading (Fig. 2). Optimal cup position is likely design specific.
In a similar study, Underwood et al. described low clearance to be a significant risk factor for edge loading and high wear. However, our results do not support this, with clearance accounting for <5% of the variation in wear rates and ion levels. While in theory lower clearance does increase the size of the contact area and therefore the risk of edge contact, the variation in clearance among designs is small and not likely significant. Low clearance also provides improved distribution of contact pressures.
CPRD explained up to 70% of the variability in wear rates and ion levels, significantly more than for any other variable. Cup inclination, which is widely recognized as the most important determinant of edge loading, only accounted for up to 17% of the variability in these outcomes. Our results show how the 3D interaction of multiple component position and design variables combine to account for a large portion of the variation in wear rates and ion levels. Our work builds on that by Underwood et al. who found that edge loaded retrievals had a significantly lower mean “contact patch edge to rim (CPER) distance.”
Although CPRD explained a significant proportion of the variability in wear, ∼30–50% remained unexplained. This may be due in part to variation in activity patterns and suboptimal positioning of the femoral component. Mellon et al. previously described an association between edge loading and patient-specific motion patterns. Although likely to be less influential than cup position, femoral position variables, such as horizontal offset and version likely affect the risk of edge loading. Although a previous study found no association between femoral version and wear rates, this remains poorly understood. Future work may focus on extending our CPRD model to include femoral positional variables. Additionally, some of the variation in wear rates may be due to other causes of edge loading such as impingement or micro-separation. Both mechanisms may cause edge wear in the presence of an acceptable CPRD, particularly anterior impingement, which is more likely to occur in MOM HRs due to the retention of femoral bone and if the cup is implanted with a shallow inclination angle or a neutral/negative version angle.
CPRD was more strongly correlated with wear rates than ion levels. Some patients with high wearing hips likely developed symptoms limiting their activity. While they had a highly worn hip (and low CPRD), their limited activity prior to revision surgery may have facilitated clearance of Co and Cr ions from the blood. Our results show that low CPRD can predict high wear in MOM-HR. However, further evaluation of the predictive value of low CPRD for raised ion levels and clinical failure in a large prospective cohort of patients is essential.
We acknowledge limitations to this study. First, like all retrieval studies our results are not representative of the entire MOM population. However, we investigated a large number of cases, and the numbers of each hip design reflected the market distribution in the period during and prior to collection. The variation and number of designs is unlikely to contribute significant bias. We investigated design features such as “arc of cover” and clearance in isolation and as part of the CPRD model. In combination with the use of multiple regression analyses we could reduce the confounding effect of multiple designs and identify design features associated with high wear. While CPRD accounts for many differences in component design, one aspect that was not investigated was the role of manufacturing variations, such as heat treatment, on wear. Whilst this is the most comprehensive model described in the literature to calculate the CPRD, we acknowledge that it is likely incomplete, and future work may identify other variables that can be incorporated in the model. An advantage of our method is the use of 3D CT scanning to measure cup inclination and version. In large diameter MOM hips, measuring cup position (particularly version) from plain radiographs is challenging due to the large metal head obscuring the margins of the cup.
This study has quantified the relative contributions of surgical and design variables on the wear performance of MOM-HR, and demonstrated how CPRD can predict high wear in MOM-HR. This work contributes to our understanding of failure and may help facilitate the development of safer implant designs. These results are clinically significant given the large worldwide burden of patients implanted with MOM-HR devices over the past two decades and the continued use of well-performing resurfacing designs such as the BHR in selected patients.